This invention generally relates to spread spectrum time division duplex (TDD) communication systems. More particularly, the present invention relates to a system and method for controlling transmission power within TDD communication systems.
FIG. 1 depicts a wireless spread spectrum time division duplex (TDD) communication system. The system has a plurality of base stations 301-307. Each base station 301 communicates with user equipments (UEs) 321-323 in its operating area. Communications transmitted from a base station 301 to a UE 321 are referred to as downlink communications and communications transmitted from a UE 321 to a base station 301 are referred to as uplink communications.
In addition to communicating over different frequency spectrums, spread spectrum TDD systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective chip code sequences (codes). Also, to more efficiently use the spread spectrum, TDD systems as illustrated in FIG. 2 use repeating frames 34 divided into a number of time slots 361-36n,, such as fifteen time slots. In such systems, a communication is sent in selected time slots 361-36n using selected codes. Accordingly, one frame 34 is capable of carrying multiple communications distinguished by both time slot 361-36n and code. The use of a single code in a single time slot is referred to as a resource unit. Based on the bandwidth required to support a communication, one or multiple resource units are assigned to that communication.
Most TDD systems adaptively control transmission power levels. In a TDD system, many communications may share the same time slot and spectrum. When a UE 321 or base station 301 is receiving a specific communication, all the other communications using the same time slot and spectrum cause interference to the specific communication. Increasing the transmission power level of one communication degrades the signal quality of all other communications within that time slot and spectrum. However, reducing the transmission power level too far results in undesirable signal to noise ratios (SNRs) and bit error rates (BERs) at the receivers. To maintain both the signal quality of communications and low transmission power levels, transmission power control is used.
One approach using transmission power control in a code division multiple access (CDMA) communication system is described in U.S. Pat. No. 5,056,109 (Gilhousen et al.). A transmitter sends a communication to a particular receiver. Upon reception, the received signal power is measured. The received signal power is compared to a desired received signal power. Based on the comparison, a control bit is sent to the transmitter either increasing or decreasing transmission power by a fixed amount. Since the receiver sends a control signal to the transmitter to control the transmitter's power level, such power control techniques are commonly referred to as closed loop.
Under certain conditions, the performance of closed loop systems degrades. For instance, if communications sent between a UE 321 and a base station 301 are in a highly dynamic environment, such as due to the UE 321 moving, such systems may not be able to adapt fast enough to compensate for the changes. The update rate of closed loop power control in a typical TDD system is 100 cycles per second which is not sufficient for fast fading channels. Accordingly, there is a need for alternate approaches to maintain signal quality and low transmission power levels.